1. Field of the Invention
The invention relates to a closure apparatus for a housing, in which a mechanically commutated electric motor is arranged at least partially, and to a servo drive which has this closure apparatus.
2. Related Art
Owing to very stringent requirements in respect of electromagnetic compatibility (EMC requirements), mechanically commutated electric motors nowadays need to have interference suppression, which is complex. This applies in particular to mechanically commutated electric motors used in motor vehicles. In the case of mechanically commutated electric motors, as a result of the constant interruption to the current flow on further-rotation of an armature, electromagnetic interference occurs, which is emitted by the electric motor to the surrounding environment. For interference suppression of the mechanically commutated electric motors, so-called rod core inductors are usually used, which can be produced inexpensively. This type of inductor damps both common-mode and normal-mode interference that can develop in the mechanically commutated electric motor. Common-mode interference is understood here to mean interference currents that flow in the same direction on all supply lines of the mechanically commutated electric motor. Normal-mode interference is understood here to mean electric currents that occur as currents flowing in both directions in the lines of the mechanically commutated electric motor. In the case of mechanically commutated electric motors, currents with very different current intensities can occur. Thus, in the case of mechanically commutated electric motors, extremely high blocking currents can occur with a current intensity, which is a multiple greater than a no-load current intensity. The blocking current characterizes a maximum current drawn by the electric motor when the rated voltage is applied if the axis of the electric motor is blocked. In many control engineering applications, the mechanically commutated electric motor is in one position for a large portion of its use time and in the process only draws a small current. Only in the case of rapid changes in position do brief current peaks in the incoming and outgoing lines of the mechanically commutated electric motor occur, wherein these current peaks can have current intensities that are higher by a factor of 10 than a mean current of the electric motor. Precisely in operating states in which these current peaks occur, electromagnetic interference results.
In order to suppress the electromagnetic interference, rod inductors can be used, which have an inductor coil core and an inductor coil wire wound around that inductor coil core. In general, the inductor coil core comprises or consists substantially of a soft-magnetic ferrite, whose relative permeability μr is substantially greater than 1. Typically, the relative permeability μr is approximately 250 in order to increase the inductance and therefore the damping, wherein the inductance increases nonlinearly with the permeability. The cause of the nonlinear relationship between the relative permeability and the inductance lies in the principle of shearing. If the inductor coil core is caused to enter saturation owing to high current peaks, the permeability of the inductor coil core can collapse and return to a value of approximately 1. This can result in a drastic dip in the damping properties and therefore a severely reduced interference suppression effect of the inductor coil. For this reason, the inductor coil core is selected to be as large as possible, depending on the amount of physical space available. In addition, the inductor coils are arranged removed from one another in such a way that they have no or virtually no magnetic coupling. In the case of additive superimposition of the magnetic fields of inductor coils, an inductor coil core can enter saturation already at very low currents, as a result of which a desired interference suppression effect of the inductor coils may be absent.